X-ray tube



1934- w. D. COOLIDGE 1,946,312

XRAY TUBE Original Filed Oct. 18 1927 Inventor: William D. Coolidge,

bg WW His Abbor'neg.

Patented Feb. 6, 1934 X-RAY TUBE William D. Coolidge, Schenectady, N.Y., assignor to General Electric Company, a corporation of New YorkOriginal application October 18, 1927, Serial No.

Divided and this application March 19, 1931. Serial No. 523,823

1 Claim.

The present invention relates to X-ray tubes and more particularly toX-ray tubes adapted to operate at voltages which are low, compared withthe voltages ordinarily employed with X-ray tubes.

The present application is a division of my copending application SerialNo. 227,027, filed Ootober 18, 1927 and entitled X-ray tubes.

In the use of low voltage X-ray tubes, it is necessary to employ awindow which is more transparent to X-rays than the lime glass employedin the ordinary X-ray tubes. Although glass, such as the so-calledLindemann glass might be employed, it is diificult to make a glasswindow of the required diameter which is less than 12 mils in thicknessand which has uniform thickness. I have found it advantageous,therefore, to employ a thin metal disc consisting of a ferro-chromealloy as the window of the tube. A window of this type having athickness of about one-half mil is, for 8000 volt X-rays, slightly moretransparent than a 12 mil Window of Lindemann glass and for 4000 voltX-rays such a metal window is much more transparent than the Lindemannglass.

A metal window is more desirable than glass since metal can be rolled toa definite and uniform thickness providing in this manner an X-ray output which is uniform in all tubes having equal excitation. Metalwindows, however, have the disadvantage that the allowable energy inputof the tube may be limited due to the heating of the window bybombardment of electrons reflected from the target.

It is one of the objects of the present invention to provide an X-raytube having a metal window which is protected from bombardment byrefiected electrons.

The novel features which I believe to be characteristic of the inventionare set forth in the appended claim. The invention itself, however, willbest be understood from reference to the following specification whenconsidered in connection with the accompanying drawing, in which thefigure is a view, partly broken away, and showing partly in section andpartly in elevation, an X-ray tube embodying the features of myinvention.

Referring to the drawing, I have indicated in Fig. 1 an X-ray tubecomprising a glass portion 1 and a metal portion 2, the latter portionbeing provided with a thin metal window 3, very transparent to X-raysand consisting of a ferro-chrome alloy containing about 25 chromium andhaving a thickness of about one-half mil. Mounted in the metal portionof the tube and adjacent the window 3 is a cathode 4 and an anodebetween which a suitable potential may be applied, this anode beingprovided with a target 5.

In the arrangement as thus far disclosed, if the metal portion 2 of thetube is metallically connected to window 3 and a voltage applied to theelectrodes of the tube, electrons will be reflected from target 5 towindow 3 causing the latter to become heated and thereby limiting theenergy input which may be supplied to the tube. To overcome thisdimculty, a metal grid 6 is mounted between the anode or target 5 andthe metal win dow. Grid 6 may be metallically connected to the cathodeelement or insulated therefrom and from the anode, as desired. If thegrid is insulated from the cathode and anode it will at first bebombarded by electrons until it has acquired cathode potential. The gridwill thereafter electrostatically prevent electrons from going to thewindow and will cause electrons which would otherwise go there to bedeflected so that they will strike some other portion of the metalenvelope 2 or be turned back on the target itself. Substantially thesame action takes place when the grid is metallically connected to thecathode. In the latter case the grid is initially at cathode potentialand will electrostatically prevent electrons from going from the targetto the metal window.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is,

An X-ray tube including an envelope and containing an anode and acathode, said tube having a metal window electrically connected to saidanode and transparent to X-rays, and means for electrostaticallyprotecting the window from electron bombardment, said means comprising agrid positioned intermediate the target and the window, and a connectionbetween the grid and the cathode.

WILLIAM D. COOLIDGE.

